The characteristics of light emanating from an object or a material may be advantageously detected and analyzed in order to determine characteristics of the object or material under examination. For many years, spectrographic techniques have been used to perform analysis of materials ranging from human blood and other biological materials to slag from a crucible. For example, it has been known that wavelengths of light absorbed by a material, as well as the wavelengths of light emitted by a material during an excited state, such as combustion, both indicate the composition of the material. Today, analytic instruments in industrial, scientific and medical applications make widespread use of such emission spectra and absorption spectra. Another such technique is Raman spectroscopy, where, for example, the output of a mercury vapor arc may be filtered and used to excite a transparent material. As the light transmits through the material, it is scattered and undergoes a change in wavelength and a random alteration in phase due to changes in the rotational or vibrational energy of the sample. Raman scattering is a principal analytic tool in industry and science today.
Another class of analytic instruments uses fluorescence to identify materials. In such systems, an excitation source, such as a laser, is used to excite atoms or molecules, raising electrons into higher energy states. When the electrons revert back to the unexcited state, they fluoresce or emit photons of light characteristic of the excited atom or molecule. In addition, the time delay between the exciting light and the emitted light, as well as the amplitude of the emitted light, provide information about the material's composition, lifetimes, and concentration of various components. Instruments that provide this function are known as frequency domain fluorometers.
While one may now visualize a system in which the excitation source is being modified at incrementally changing rates while simultaneously the corresponding emission spectra is being measured and analyzed over time, in practice, such measurements are achieved by modulating a light source. For example, one may employ for this purpose a pulsed dye laser, or a continuous wave laser whose output is externally modulated by a Pockels cell or an LED which is intrinsically modulated. However, it is anticipated that almost any light source which can be either modulated or pulsed may be employed to implement the invention.